AST-1452 Remove hard coded phase center

Removes the hard coded phase center and uses the phase center from the measurement set instead.

This meant that some code in dp3_helper.py could be removed, because in some cases if no direction is specified, DP3 will use the phase center from the measurement set by default. The shift step in DP3 changes the phase center. When the direction the MS is shifted to, is the phase center in the MS, then the shift does nothing, and can be skipped altogether.